The manufacture of certain types of display cells calls for the construction of glass envelopes formed by sealing together two glass sheets with a perimeter seal. For some applications, such as in the construction of liquid crystal display cells, the thickness of the interior space of the envelope is a relatively critical parameter. Up to a certain size of liquid crystal display cell adequate control of this thickness is given solely by control of the thickness of the perimeter seal. It will be apparent, however, that if the cell size is to be increased, the flatness of the two sheets will have to be held to correspondingly closer tolerances. At a certain stage these tolerances can only be met by optically working the surfaces of the two sheets. This necessarily involves using relatively thick sheets, and this is undesirable for a number of applications. For instance, in a liquid crystal display cell operated in a reflective mode it is liable to involve separating the reflecting surface from the liquid crystal material by such a distance as to restrict substantially the viewing angle because of the parallax effects between the liquid crystal layer and its image in the reflecting surface. Some of these disadvantages are removed, or at least ameliorated, if at least one of the sheets is relatively thin.